High temperature thermal processes, for example, the generation of steam for the production of electricity in power plants utilizing fossil fuels and biomass or the incineration of domestic waste, often create environmentally harmful by-products. These, among others, are fly ash, nitrogen oxide (NOx—refers to NO and NO2), and sulfur dioxides (SO2 and SO3). These compounds have to be removed from the flue gases of the high temperature thermal process before being discharged to the environment.
Desulphurization of the flue gas, for example removal of SO2, may be carried out by applying known methods in which the SO2 produced in the combustion process is oxidized to SO3. This is done prior to exposure of the flue gases to the reduction catalyst. The SO3 may then be absorbed into an alkaline solution and removed from the process, usually in the form of gypsum (calcium sulfate).
The standard for removing NOx from flue gases is the Selective Catalytic Reduction (SCR) process, where a reducing reagent, typically ammonia, is injected, mixed into the flue gas, and sent through a catalytic reaction chamber where a catalytic converter facilitates the reduction of NOx with the reducing agent to form nitrogen gas and water. The catalytic converter is typically made of a substrate and a nitrogen oxide reducing catalyst. The substrate can generally be of any appropriate material for supporting a catalyst, such as metals or ceramics. In plate-type configurations, for instance, the substrate can comprise the metal mesh (i.e., a metal substrate) on which titaniumdioxide (for example) in conjunction with a other compounds are supported and build a ceramic body or carrier material with a defined porous structure. As such, plate-type configurations include both a metal and ceramic substrate. Honeycomb- or corrugated-type configurations, however, do not utilize metal substrates. Instead, honeycomb- or corrugated-type configurations utilize titaniumdioxide (for example) in conjunction with other compounds to form monolithic structures known as ceramic substrates. In both cases, however, the nitrogen reducing catalyst (e.g., vandiumpentoxide, tungstenoxide, and molybdenumoxide) is embedded in or deposited on the titaniumdioxide. Moreover, the catalyst is embedded in or homogenously distributed throughout the substrate only during the initial production process.
Catalytically relevant metal compounds are added to the dough prior to extruding the catalyst. During the subsequent calcinations process the catalytically relevant substances are converted to the catalytic metal oxides (e.g., vandiumpentoxide, tungstenoxide, and molybdenumoxide). In some cases, however, a manufacturer may choose not to add the catalytically relevant substances to the dough but rather impregnated/soak the substrate in catalytically relevant substances followed by calcinations. In some cases a manufacturer may also add an impregnation step to selectively deposit the nitrogen reducing catalyst on the substrate. In other cases, however, a manufacturer may omit adding nitrogen reducing catalyst to the dough but impregnate the nitrogen reducing catalyst onto the substrate.
Throughout the operation of the catalyst, it becomes contaminated due to the accumulation of various substances from the flue gas on the catalyst. Most of them are responsible for the catalyst's decrease in activity such as Sodium, Potassium, Phosphorus and Arsenic. Others, like Iron, however, are known to be the main contributor for the increase of the SO2/SO3 conversion rate during the catalysts usage cycle. This type of contamination is due to chemical bonding of the compounds onto the catalyst. Iron contamination may come from a variety of sources, including the fuel burned in the power plant. For example, depending on the origin and age of coal, the natural iron content may range from about 5% to about 8% by weight, relative to the total amount of the mineral components in the coal.
It is generally known that during the regeneration of SCR catalysts, inorganic acids, such as sulfuric acid (H2SO4) and hydrochloric acid (HCl), may be used to clean and restore the catalyst, such as by a soaking step and a neutralizing step. Inorganic acids are typically odorless, which is another advantage to their use. Sulfuric acid, in particular, is relatively inexpensive and commercially available. In addition, sulfuric acid is known to not have any negative impact on the catalyst. However, treating a catalyst with sulfuric acid has disadvantages since the H2SO4 in a diluted aqueous solution also corrodes the steel casings of the catalyst. Corrosion of the catalyst casings may also result in release of water soluble iron compounds that can penetrate the pores of the SCR catalyst, further enhancing the undesired SO2 to SO3 conversion process.
The removal of iron contaminants from a DeNOx catalyst has been described in U.S. Pat. No. 7,569,506 in which the catalyst is placed in a reaction solution comprising an aqueous solution of an inorganic or organic acid with the addition of one or more antioxidants. Inorganic acids, namely hydrochloric acid, phosphorus acid, nitric acid, and, in particular, sulfuric acid, are described. Organic acids, such as relatively strong organic acids, including oxalic acid, citric acid, malonic acid, formic acid, chloroacetic acid, and benzole sulfonic acid were also used. Although the methods described in this reference were effective in removing iron accumulation on the catalyst, the strong acids described in the reference also liberated iron ions from steel substrates and the steel casings of the catalyst. These iron ions can then penetrate the pores of the catalyst, potentially enhancing the undesired SO2 to SO3 conversion.
Accordingly, there remains a need for alternative methods for removing iron (e.g., iron compounds) accumulated on a catalytic converter to remove contamination by iron compounds and provide optimum performance of the catalytic converter while minimizing or reducing the SO2 to SO3 conversion process within the flue gas stream.